Attempts have been made in the art to provide a single antenna system that is suitable for transmitting and/or receiving radiations in more than one frequency spectrum or wavelength range. For example, the article "Multispectral (IR/MMW) Dome For seeker Application" by G. Forti, W. Menicacci--S.M.A., A. Novi, G. Simoncini, and L. Calami, OFF. GALILEO, Florence, Italy, pages 157 to 166, as published in the Sixth European Electromagnetic Structures Conference Proceedings, Friedrichshafen, Germany (1991), discloses a rotationally symmetrical Cassegrain-Antenna arrangement which is schematically shown in present FIG. 9. This conventional reflector antenna arrangement includes a primary or main reflector 101 and a secondary or subreflector 102 arranged symmetrically and coaxially along a common radiation axis CA. Infrared (IR) active elements 104 as well as millimeter wave radiation (RF) active elements 103 are integrated together into a feed horn arranged along the common axis in a typical Cassegrain arrangement. Due to the incorportion of both IR elements 104 and RF elements 103 into the feed horn, both the IR radiation and the RF radiation will be reflected in the same manner along the same path from the subreflector 102 and from the main reflector 101, and in common along the common axis CA.
With the above described known arrangement, the infrared (IR) signals suffer undesirable side effects, especially in some applications, because it is impossible to optimize the reflectors simultaneously both for the RF radiation and for the IR radiation. For this reason, it becomes necessary to provide a directly radiating aperture for the IR radiation. Such a directly radiating aperture, however, cannot be provided in a system in which the radiation directions or radiation axes of the two radiation spectra are coincident along the common axis CA, because both the exciter as well as the subreflector would necessarily be located in the beam path and thus in the way of a direct radiation of the IR radiation. Furthermore, it has been a continuing problem for conventional antennas to achieve a space-saving integration of a millimeter wave sensor system for RF radiation and an infrared system for IR radiation in a particularly compact arrangement, for example using an offset double reflector antenna arrangement. Particularly, it has not been possible to achieve an acceptable antenna system having the following characteristics: a direct radiating infrared aperture without interference, the smallest possible degradation of the millimeter wave RF signal path, and a coincidence of the radiation beam axes of the two systems externally from the antenna arrangement.